Suppose that a world-class swimmer determined that he wanted to break the world record for the longest unassisted ocean swim. On the day that he attempted to break the record, a boat followed him. In the boat were a rescue swimmer, a paramedic, someone to officially witness the record, and a camera man. As the swimmer approached the record breaking mark, his muscles were strained beyond anything that he had ever known before, but he mustered the strength and determination to keep swimming. After countless hours, he finally broke the record.
Now, if you think about it, the presence of the boat virtually guaranteed that he could not have drowned. Even so, he had to break the record without the assistance of anyone in the boat. And he did just that. The record was broken by his own blood, sweat, and tears. The fact that the boat was there did not make the experience any less grueling. No reasonable person could say "the record doesn't mean anything because of the boat," or "since he couldn't have drowned, he doesn't understand what it is like to be a real long distance swimmer."
I believe that the temptation of Christ, while not perfectly parallel (no analogy is perfect), is comparable in many ways to this scenario. In the same way that the boat ensured that the swimmer could not drown, Christ's divine nature ensured that he could not have sinned. However, Jesus is also truly human, and he resisted the temptation of sin without having to rely on "the boat." And I think that is really the big concern of those who want to say that Jesus could have sinned. If the swimmer could have simply jumped in the boat and cruised when the going got tough, then the record would mean nothing. And I think that is a fair assessment. That is why I think that something similar to the swimmer/boat paradigm was at work in Jesus of Nazareth. In the Garden of Gethsemane, Christ experienced terrible anguish as he contemplated what was required to remain obedient to the Father. That doesn't make any sense if he was "just hanging out in the boat," so to speak. The boat didn't make the swimmer's muscles work or keep them from getting sore. Even though he couldn't have drowned, he knows the pain very well. If anything, he knows it better than anyone else. In the same way, Christ's divine nature did not make him obey or take away from how difficult it was. Even though he couldn't have sinned, he knows how hard it is resist temptation--to be obedient. If anything, he knows it better than anyone else. Thus, Hebrews 4:15 holds true. He "understands our weaknesses, for he faced all of the same testings we do, yet he did not sin" (NLT). And the fact that he could not have sinned does not detract in any way from the significance of his sinlessness.
*Some suggest that this is not a genuine problem because Jesus was also human. They say that "as God he couldn't sin, but as a human he could." The reason I don't go that route is because I think it is incorrect to talk about the two natures of Christ as if they are two different people rather than a union. I certainly acknowledge that there is a unique interaction between his natures such that he was able to live as a man (let alone as a baby and child), but I don't think we can just disconnect the two so as to say that Jesus the man could have sinned, but Jesus the second person of the Trinity could not.
**I didn't come up with the swimmer analogy. A quick search for "swimmer analogy and Christ's temptation" gives Bruce Ware credit for it in his book, The Man Christ Jesus: Theological Questions on the Humanity of Christ published by Crossway, but that is not where I heard it from.